A.C.
Have you thought about taking the Amtrak train anywhere? We live in Burleson and used to take the train up to OKC. It sounds boring, but my kids LOVED it really. It's 3/4 hours by car and 4 hours by train to get there. You go through some really prett country on the train, and there's enough room for the kids to get up and walk around. On the way up it's at night, and mine fell asleep and on the way back it's during the day so they could see more. It takes you to downtown OKC where they've really done a good job fixing up Bricktown. It's with in walking distance of a few different hotels, there's a nice canal area with food choices, a playground, one of those big fountains that spring up in the ground, and a movie theatre. But, we'd enjoy just walking around down there, going to the play ground, the kids would play in the fountain, etc. The last time we went my kids were about the same age as yours now (they are 3 and 5 now).
Fredricksberg I think it more for adults than kids. But Austin is bound to have stuff for the kids to do. San Antonio is also not that far really, which has a lot of choices as well. Hope you find something!